1. Field of the Invention
The invention relates to a fingerprint detection device and a method of its manufacture. More particularly, the invention relates to a fingerprint detection device having a semiconductor chip for detecting a fingerprint (hereinafter referred to as fingerprint detectable semiconductor chip), the outermost upper surface of the semiconductor chip covered with a protective film of a carbon based material (the protective film hereinafter referred to as carbon based protective film) which can enhance the intensity of the surface and the electrostatic discharge withstand voltage of the semiconductor chip without any protective cover. The invention also relates to an apparatus for forming such protective film.
2. Description of the Related Art
In recent years, there are many occasions in banks and government offices for example where a fingerprint detection device is used to identify a person, to allow operation of an electronic device, and to authorize entrance to a specific facility.
Fingerprint detection devices for such uses as mentioned above have sense electrodes arranged in a grid on a semiconductor substrate and a protective film for covering the surface of the sense electrodes. These semiconductor chips are also called fingerprint sensor chips. When a finger is placed on the protective film, condensers are formed between a surface of the finger and the sense electrodes, with the protective film serving as a dielectric. The capacitance, and hence the voltage of each of these condensers vary from point to point, in accordance with the protrusions and recesses, i.e. ridges and troughs, of the fingerprint. Thus, by detecting the voltage distribution, a pattern indicative of the fingerprint can be detected (see, for example, U.S. Pat. No. 5,325,447).
In order to permit a finger to directly touch the surface of the semiconductor chip of the fingerprint detection device, the surface of the chip must be exposed. Therefore, the protective film must have strength to withstand such physical attack as scratching by fingers, nails, coins, pencils and the like. Besides the strength against such physical attacks as mentioned above, the protective film is also required to have resistance against static electricity. In addition, the capacitances must be easily detected. Specifically, the protective film is required to have a small friction coefficient, good hardness, high dielectric breakdown strength, high melting point, high thermal conductivity, small thermal expansion coefficient, and large specific dielectric constant.
In conventional fingerprint detection devices, a silicon nitride film is formed on the outermost surface of the semiconductor chip. The silicon nitride film serves as a protective film and dielectric layer of a respective condenser.
However, silicon nitride film has hardness on the order of 9–10 GPa, which is not hard enough to be a scratch-proof protective film. In order to avoid exposing the fingerprint detectable semiconductor chip (also hereinafter referred to as semiconductor chip), a metal cover is commonly used to protect the semiconductor chip. However, such cover that must be opened and closed each time is, at least mentally, unfavorable. Moreover, opening/closing mechanism of the cover hinders fabrication of a fingerprint detection device in a thin and/or compact form, and also hinders reduction of the manufacturing cost.
This is a serious problem especially when a fingerprint detection device is mounted on a portable electronic apparatus for enhancing its personal identification function.